11:50 a.m. Update:
An "electrical disturbance caused a loss of power to generator cooling equipment" at the plant, Exelon Nuclear spokesperson Dana Melia said in a statement.
"There is no threat to the health and safety of the public associated with this event," Melia said.
The company said the Unit 1 plant will remain offline until repairs, testing, and inspections are complete. The generating station's other power plant, Unit 2, continued to operate at full power.
The plant, which is in the process of applying for license extensions that would keep it operating until 2049, experienced three unscheduled shutdowns between February and June of 2011.
The Unit 1 nuclear reactor at Exelon's Limerick Generating Station was shut down at 8:15 a.m. today following an electrical fault in a transformer, the Nuclear Regulatory Commission said in a statement sent to area media outlets.
The reactor was manually shut down from the power station's control room following the fault, according to the NRC.
The Limerick plant is located about 40 miles from Allentown in Montgomery County, northwest of Philadelphia. In the event of a serious problem, the plant's evacuation zone would extend about 10 miles.
"The NRC is closely monitoring the shutdown, with our Resident Inspectors assigned to the plant gathering information from the control room and sharing it with NRC staff. There do not appear to be any complications at this point. We will want to know more about Exelon’s root cause evaluation of what caused the event and about any and all corrective action," NRC spokesperson Neil Sheehan said.
The NRC has two inspectors permanently assigned to the facility.